The present invention relates to a process for preparing a hydrocarbon reforming catalyst and particularly to a process for preparing an improved catalyst for reforming naphtha fraction to obtain a high-octane gasoline or aromatic hydrocarbons.
More particularly, the present invention is concerned with a process for preparing a platinum-rhenium catalyst to be used for a catalytic reforming naphtha fraction, in which a platinum carbonyl cluster-anion complex salt is used as platinum and a rhenium compound is used as rhenium.
Catalytic reforming reaction is one of important reactions in the petroleum refining and petrochemical industries. The most important reaction in this process is an aromatization reaction, in which are included dehydrogenation or isomerization-dehydrogenation reaction of cycloparaffins and dehydrocyclization reaction of paraffins. There also occur other reactions such as isomerization, hydrocracking and hydrogenolysis. Particularly, hydrocracking and hydrogenolysis reactions produce lower hydrocarbons and reduce the yield of gasoline and that of aromatic hydrocarbons, and therefore it is desirable that the occurrence of these reactions be minimized.
For catalytic reforming, platinum-halogen-alumina catalysts were mainly used during the period between 1950 and 1960, but from the latter half of 1960 those catalyts were replaced by catalysts containing rhenium, iridium or germanium as the second component in addition to platinum, and at present almost all catalytic reforming apparatus are filled with bimetallic catalysts, typical of which is a platinum-rhenium catalyst. For example, in Japanese Patent Publication No. 31859/1970 there is disclosed a reforming process for naphtha fraction using a platinum-rhenium catalyst, in which chloroplatinic acid, ammonium chloroplatinate or polyamine platinum salt is used as the platinum source. And in Japanese Patent Publication No. 29362/1972 there is disclosed a reforming process for naphtha using a platinum-rhenium bimetallic catalyst, in which chloroplatinic acid, ammonium chloroplatinate or polyamine platinum salt is used as the platinum source. According to these processes, the resultant catalysts are superior in oxidation stability, but it is necessary that the feedstock fed to the catalytic reforming process be substantially free from sulfur and go through a desulfurization treatment before reforming. If the sulfur content of the feedstock is 1 ppm or more, the catalyst stability will be lost repidly and the intrinsic performance of the platinum-rhenium catalyst will not be exhibited. For the desulfurization treatment, usually a hydro-refining equipment is used, but considerable expenses are required for the construction and operation of this equipment.
Further, in Journal of Chemical Society Chemical Communication (J.C.S. CHEM. COMM.), 11 (1976) there is disclosed a selective dehydrocyclization reaction of n-hexane which is performed over supported platinum crystals produced from a platinum carbonyl cluster-anion complex salt. The catalyst used therein is prepared by supporting on .gamma.-alumina a platinum carbonyl cluster-anion complex salt of the general formula [Pt.sub.3 (CO).sub.6 ].sub.n.sup.2- [NEt.sub.4 ].sub.2 (n=2.about.5) according to an impregnation method. That catalyst exhibits a good stability of its activity even when the sulfur content of the feedstock is 1 ppm or more, but is inferior in catalyst performances such as activity, aromatization selectivity and stability in comparison with a platinum-chlorine-alumina catalyst prepared by a conventional method and having the same platinum content and a commercially available platinum-chlorine-alumina catalyst. As to the activity, selectivity and stability used herein, (1) the activity means a measure of conversion of starting hydrocarbons into products under specific reaction conditions, (2) the selectivity means the amount of a specific product relative to converted starting hydrocarbons and (3) the stability means a rate of change relative to time of the activity and selectivity or of a value (yield) obtained by multiplying the two; the smaller this rate, the better the stability. The above catalyst is inferior also in its stability against oxidation.